The present disclosure pertains to an optical device and a method for controlling a refractive index profile in the optical device.
Fiber lasers can be used in many applications and are increasingly sought after in certain applications as substitutes to solid state lasers. Fiber lasers are more compact, reliable, can produce a good quality beam, have high efficiency, and can provide robust single-mode output. In addition, fiber lasers can be used in an all-fiber architecture without free-space optics and hence may not require a rigid optical bench. Over the past decade, output powers of fiber lasers have been increased several orders of magnitude, from the watt-level to multi-kW powers, making fiber lasers competitive with solid state based lasers.
The limits in scaling up power in fiber lasers include nonlinear effects and optical distortions due to heat generated within the laser medium. Reliable, long-term operation requires that the fiber be cooled. In addition, heat flow in the fiber medium can establish a fast-axis temperature gradient within the fiber. Temperature gradients can also arise due to non-uniform cooling, heat spreading, and several other factors. These temperature gradients can introduce a varying transverse refractive index profile due to the fact that, generally, the refractive index depends on the local temperature. Conventional low power fiber amplifiers can be immune to such transverse index gradients, since the variation in index profile of the fiber material (built-in index profile) is generally greater than the thermally induced variation in the index. However, in some high-power designs, the built-in index profiles are designed to be much weaker than in conventional fibers. As a result, the thermally induced transverse index profiles can be problematic.
In a fiber laser, accumulated thermal lensing due to variation in the index profile can create strong refraction effects which can result in a large radiative loss that can significantly degrade the laser efficiency and output beam quality.